An optical pickup must trace accurately tracks of 1.6 μm intervals in an optical disk such as a CD. Accordingly, in order to ensure stable signal detection, control at one-tenth or less a track width is required, and a control operation (tracking servo) is generally performed with a tolerance of 0.1 μm or less.
In an optical pickup device, this control operation is performed by moving an objective lens back and forth in a horizontal direction, and it is necessary to detect a signal (tracking error signal) which indicates a direction (right or left) and an amount of the movement.
Now, a three-beam method or a push/pull method has been known as a tracking servo system of an optical pickup device.
FIG. 8 is an explanatory drawing of the three-beam method. As shown in FIG. 8, a main beam SP11 is converged on an optical disk 101. On the both sides of the main beam SP11 are converged subbeams SP12 and SP13 for detecting a tracking error signal.
In this method, when the converged main beam SP11 traces accurately the track, output of the subbeam SP12 is equal to that of the subbeam SP13. On the other hand, when a tracking deviation occurs in converging the main beam SP11, output of the subbeam SP12 becomes different from that of the subbeam SP13.
Here, in the three-beam method, the tracking error signal is a differencial signal D11-D12 which is obtained from intensities D11 and D12 detected by a photodetector with respect to disk-reflected light of the respective subbeams SP12 and SP13.
However, in the three-beam method, the subbeams SP12 and SP13 to be a tracking error signal are spaced by a large distance. This causes a difference in quantity of reflected light between the subbeams SP12 and SP13 even when tracking is performed from a part where no information is recorded to a part where information is recorded in the optical disk 101 without the main beam SP11's tracking deviation, as shown in FIG. 8. Accordingly, with respect to the differential signal D11-D12, a problem arises that a tracking offset occurs due to the difference in quantity of reflected light, not due to a tracking deviation.
FIG. 9(a) and FIG. 9(b) are drawings for explaining the push-pull method. As shown in FIG. 9(a) and FIG. 9(b), the light emitted by a laser 111 is converged through an objective lens 112 onto a pit on an optical disk 113. The converged light is diffracted and reflected by the pit on the optical disk 113, and are incident on the objective lens 112 again. The reflected light incident on the objective lens 112 is incident via a beam splitter 114 on a dichotomized photodetector 115 to be detected. The dichotomized photodetector 115 has a photodetection face of two equally-divided areas and is set so that when the light beam is properly positioned with respect to the pit on the disk, two (right and left) intensities of reflected light respectively detected by the two photodetection areas become equal to each other.
Thus, if the light beam is properly positioned with respect to the pit, intensity distributions of the reflected light are indicated equally on the right and left areas of the photodetection face. That is, if the light beam is properly positioned with respect to the pit on the disk, the two photodetection areas (left and right) of the dichotomized photodetector 115 detects reflected light of equal intensity distribution. However, a deviation from the proper position causes asymmetric intensity distributions of the reflected light on the right and left areas of the dichotomized photodetector 115. That is, when the position of the light beam deviates from the pit, two (left and right) intensity distributions of reflected light detected by the two areas of the photodetection face become asymmetrical. Here, in the push-pull method, the tracking error signal is defined to be a differential signal D11′-D12′ which is obtained from detected intensities D11′ and D12′ of reflected light on the two detection areas (right and left).
However, the push-pull method has the following problem. In the push-pull method, a tracking error signal is produced by detection of a difference of light quantity distributions of reflected light from the optical disk 113 on the right side and left side of the dichotomized photodetector 115. That is, a tracking error signal is obtained by detection of a difference of intensity distributions of reflected light respectively detected on the two photodetection areas of the dichotomized photodetector 115. However, in the tracking of an optical disk, in case where the objective lens 112 moves in the radial direction, there are cases where a light axis of reflected light from the optical disk 113 shifts, and accordingly a beam center shifts from the center of the photodetection face of the dichotomized photodetector 115, as shown in FIG. 9(a). In addition, in case where a face of the optical disk 113 has a tilt, the reflected light from the optical disk 113 may return to the objective lens 112 with an angle, thereby shifting the center of the reflected light with respect to the objective lens 112, as shown in FIG. 9(b).
Thus, in both cases, there is a problem that despite no tracking deviation, a differential signal D11′-D12′ from the dichotomized photodetector 115 is offset to cause inaccurate tracking.
In light of this problem, Japanese Laid-Open Patent Publication No.45451/1999 (Tokukaihei 11-45451, published on Feb. 16, 1999) discloses a technique to solve this problem. FIG. 10(a) is a perspective view illustrating an optical pickup optical system of this conventional technology. As shown in FIG. 10(a), the light emitted from a semiconductor laser 121 is split by a first diffraction element 122 into three light beams, i.e., a zeroth-order light beam forming a main spot on an optical disk 127 and ±first-order light beams forming two subspots. Then, after passing through a second diffraction element 123, the three light beams are converted by a collimator lens 124 into parallel light beams. Furthermore, after passing through a beam splitter 125, these three light beams form on the optical disk 127 via an objective lens 126 a main spot SP21 based on the zeroth-order light beam and two subspots SP22 and SP23 based on the ±first-order light beams. After that, the three light beams are diffracted and reflected by a pit on the optical disk 127. Then, the reflected three light beams become parallel light beams again through the objective lens 126. Although these three parallel light beams pass through the beam splitter 125, these light beams are partially reflected by the beam splitter 125 and led to a magneto-optical signal detection system. The light beam led to the magneto-optical signal detection system is converted into electrical signals by elements such as an analyzer and a light receiving element for reproducing signals of the optical disk 127. Information recorded in the optical disk 127 is reproduced in this manner.
On the other hand, the other light beams having passed through the beam splitter 125 also pass through the collimator lens 124 and are diffracted by the diffraction element 123. The diffracted first-order light beam is received by light receiving elements 131 to 133 to be detected as a servo signal (focus error signal and tracking error signal, hereinafter referred to as FES and TES, respectively) and as an optical disk's land/groove (L/G) discriminating signal.
The FES is a signal for detecting a direction (upward or downward) and an amount of movement of the objective lens 126 in the focus direction. The TES is a signal for detecting a direction (right or left) and an amount of movement of the objective lens 126 in the tracking direction. Furthermore, a land means a raised part of the optical disk which defines the track. A groove means a recessed part of the optical disk which defines the track.
Next, in this optical system, the following will explain a positional relationship between the main spot SP21 and the subspots SP22 and SP23 on the optical disk 127, and light-receiving regions in the light receiving elements 131 to 133 for receiving light diffracted by the diffraction element 123.
As shown in FIG. 10(b), the main spot SP21 and the subspots SP22 and SP23 are placed symmetrically on the optical disk 127 at the intervals one-forth (P/4) of a track pitch. Note that, L represents a land, while G represents a groove. Therefore, a track pitch is a width of one land width and one groove width added together.
As shown in FIG. 11(a) through FIG. 11(c), the diffraction element 123 has divided three regions, i.e., regions 151 to 153. The diffraction element 123 is divided by a division line in the radial direction of the optical disk into two regions, i.e., the region 151 and the other region (152+153). Further, this other region is divided by a division line in the track direction of the optical disk into the regions 152 and 153.
Now, FIG. 11(a) shows a diffraction pattern SP22′ on the diffraction element 123 by reflected light from the subspot SP22 on an optical disk. In the diffraction pattern SP22′, among the diffracted light beams of the diffraction element 123, the light diffracted in the region 151 is not received. The light diffracted in the region 152 is received by a light receiving section C which is one of the divided sections of the light receiving element 131, while the light diffracted in the region 153 is received by a light receiving section D which is one of the divided sections of the light receiving element 133.
Next, FIG. 11(b) shows a diffraction pattern SP21′ on the diffraction element 123 by reflected light from the main spot SP21 on an optical disk. In the diffraction pattern SP21′, among the diffracted light beams of the diffraction element 123, the light diffracted in the region 151 is converged on the division line between light receiving sections G and H of the light receiving element 132. The light diffracted in the region 152 is received by a light receiving section E which is one of the divided sections of the light receiving element 131, while the light diffracted in the region 153 is received by a light receiving section F which is one of the divided sections of the light receiving element 133.
Furthermore, FIG. 11(c) shows a diffraction pattern SP23′ on the diffraction element 123 by reflected light from the subspot SP23 on an optical disk. In the diffraction pattern SP23′, among the diffracted light beams of the diffraction element 123, the light diffracted in the region 151 is not received. The light diffracted in the region 152 is received by a light receiving section A which is one of the divided sections of the light receiving element 131, while the light diffracted in the region 153 is received by a light receiving section B which is one of the divided sections of the light receiving element 133.
Now, when the outputs of light receiving sections A to H of the receiving elements 131 to 133 are denoted as As to Hs, respectively, servo signals are found by the following operations:FES=Gs-HsTES=(Es-Fs)−β[(As-Bs)+(Cs-Ds)]  (1)
(β is a constant)
L/G discriminating signal=(As-Bs)−(Cs-Ds)
(L/G: land/groove)
In TES, β is a constant which represents an intensity ratio of the main spot SP21 to the subspots SP22 and SP23.
However, the TES and L/G discriminating signal have the following problems.
Equation (1) finds TES by using that (As-Bs) and (Cs-Ds) are sine waves whose wavelengths are shifted by a phase angle of π/2 with respect to (Es-Fs), leading and lagging (Es-Fs), respectively, and by cancelling (Es-Fs) by a sum of (As-Bs) and (Cs-Ds).
However, as shown in FIGS. 12(a) and 12(b), each of (As-Bs), (Cs-Ds), and (Es-Fs) has a signal waveform in the form of a sine wave, with a period of track pitch P. A signal amplitude is shown as |As-Bs|<|Cs-Ds|. The following will explain the reason why a signal amplitude is shown as |As-Bs|<|Cs-Ds|. The diffraction pattern SP22′ in FIG. 11(a) and the diffraction pattern SP23′ in FIG. 11(c) are shifted away from each other in the track direction. In other words, the diffraction pattern SP22′ shifts toward upstream in the track direction relative to the diffraction pattern SP23′. With this, with respect to a beam diffraction area in the regions 152 and 153, the area of the diffraction pattern SP22′ is larger than that of the diffraction pattern SP23′. Accordingly, as a signal, the operation (Cs-Ds) of the light receiving elements 131 to 133 in FIG. 11(a) is larger than the operation (As-Bs) of the light-receiving elements 131 to 133 in FIG. 11(c). In addition, a difference in size of signals may also occur due to a sensitivity variation of thee light receiving elements 131 to 133, a variation of the diffraction element 123's diffraction efficiency, or a variation of the circuits performing the operations. Note that, as shown in FIG. 12(a), each of the signals in this case has the same offset component generated by a tilt of the optical disk, or by a change of beam spot positions on the diffraction element as a result of a shift of the objective lens.
Therefore, the operation in the second term of equation (1) for determining TES, that is,β[(As-Bs)+(Cs-Ds)]  (2),becomes |β(As-Bs)|<|β(Cs-Ds)|. As a result, as shown in FIG. 12(c), the operation result of (2) includes an additional residual offset due to the (Cs-Ds) operation component other than the tracking offset.
Therefore, in the operation of equation (1) in which the result of operation (2) is subtracted from a push-pull signal (Es-Fs) having a tracking offset due to the main beam, while it is possible to cancel an offset component generated by a tilt of the optical disk, or a variation of beam spot positions on the diffraction element as a result of a shift of the objective lens, a new Cs-Ds component is added as an additional residual offset.
As a result, as shown in FIG. 12(b), there is a problem that the additional residual offset is added to TES, and the signal phase of the TES shifts by ø from the proper signal phase, so that accurate tracking control cannot be performed.
Furthermore, the following will explain a problem caused by a tracking error signal when an adjustment error occurs in positioning subbeams on the optical disk when assembling and adjusting the pickup.
In a pickup which is designed to project the beams at the intervals of n/4 (n is odd number) of the track pitch, for example, one-forth (P/4) of the track pitch as shown in FIG. 6(a), or five-fourths (5P/4) of the track pitch as shown in FIG. 6(b), the following considers the case where positions of the subbeams are adjusted so as to be δ away from the ideal position. Here, explanations are given based on a principle of servo signal detection using the respective signal waveforms, with reference to FIGS. 13(a) through 13(c).
As shown in FIGS. 13(a) and 13(b), each of (As-Bs), (Cs-Ds), and (Es-Fs) has a signal waveform in the form of a sine wave, with a period of track pitch P. Here, (As-Bs) and (Cs-Ds) in the second term of equation (1) for determining TES have signal amplitudes different from that of the ideal waveform and have a phase difference of ±γ with respect to the ideal waveform, as shown in FIG. 13(a).
As a result, the operation result ofβ[(As-Bs)+(Cs-Ds)]  (2)includes a residual offset which is out of phase from (Es-Fs) as shown in FIG. 13(c).
As a result, there is a problem that the residual offset is added to the TES, and the signal phase of the TES is out of phase by øb from the ideal TES phase, so that accurate tracking control cannot performed.
Furthermore, the L/G discriminating signal is found by the operation of (As-Bs) and (Cs-Ds). However, since (As-Bs) and (Cs-Ds) are out of phase, as shown in FIG. 13(c), the phase of the L/G discriminating signal is out of phase by øc from the ideal discriminating signal, so that accurate land/groove discrimination cannot be performed. Accordingly, the tracking servo system cannot perform a pull-in operation.
Incidentally, the residual offset tends to become larger as the adjustment error becomes large in positioning subbeams on the optical disk when assembling and adjusting the pickup. Therefore, conventional pickups must be assembled so that an adjustment error in positioning subbeams becomes as small as possible. This means greater difficulties in assembling and lower yield, resulting in rise in manufacturing cost.